Nonaqueous secondary batteries, such as lithium ion secondary batteries, have been widely used as power supplies for portable electronic devices such as laptop computers, mobile phones, digital cameras, and camcorders. Further, these batteries are characterized by having high energy density, and thus the application to automobiles and the like has also been studied in recent years.
With the reduction in size and weight of portable electronic devices, the outer casing of a nonaqueous secondary battery has been simplified. At first, a battery can made of stainless steel was used as an outer casing, and then an outer casing made of an aluminum can was developed. Further, a soft pack outer casing made of an aluminum laminate pack has also been developed nowadays. In the case of a soft pack outer casing made of an aluminum laminate, because such an outer casing is soft, a space may be formed between an electrode and a separator during charging and discharging, causing a technical problem in that the cycle life is reduced. In terms of solving this problem, a technique for attaching an electrode and a separator together is important, and a large number of technical proposals have been made.
As one of the proposals, a technique of using a separator including a polyolefin microporous membrane, which is a conventional separator, and a porous layer made of a polyvinylidene-fluoride-based resin (hereinafter sometimes referred to as an adhesive porous layer) formed thereon is known (see, e.g., Patent Document 1). When such an adhesive porous layer with an electrolyte contained therein is stacked on an electrode and heat-pressed, the electrode and the separator can be well joined together, allowing the adhesive porous layer to function as an adhesive. As a result, it is possible to improve the cycle life of a soft pack battery.
In addition, in the case where a battery is produced using a conventional metal can outer casing, electrodes and a separator are stacked together and wound to produce a battery element, and the element is enclosed in a metal can outer casing together with an electrolyte, thereby producing a battery. Meanwhile, in the case where a soft pack battery is produced using a separator like the separator of Patent Document 1 mentioned above, a battery element is produced in the same manner as for the battery having a metal can outer casing mentioned above, then enclosed in a soft pack outer casing together with an electrolyte, and finally subjected to a heat pressing process, thereby producing a battery. Thus, in the case where a separator including an adhesive porous layer as mentioned above is used, it is possible to produce a battery element in the same manner as for the battery having a metal can outer casing mentioned above. This is advantageous in that there is no need to greatly change the production process for conventional batteries having a metal can outer casing.
Against the background mentioned above, various technical proposals have been made in the past for separators made of a polyolefin microporous membrane and an adhesive porous layer stacked thereon. For example, in terms of achieving both the ensuring of sufficient adhesion and ion permeability, Patent Document 1 presents a new technical proposal focusing on the porous structure and thickness of a polyvinylidene-fluoride-based resin layer.